1. Field of the Invention
The invention relates to temperature indicating compositions and more particularly relates to liquid crystal compositions, their use and temperature sensing devices employing them as indicator compositions.
2. Brief Description of the Prior Art
Temperature sensing devices including temperature recorders employing cholesteric liquid crystal compositions as the temperature sensitive indicator are generally well known; see for example U.S. Pat. Nos. 3,704,625 and 3,974,317. Such devices function by virtue of a color change exhibited by the liquid crystal indicators under certain temperature conditions. Liquid crystals are compounds which possess a so-called mesomorphic phase (sometimes referred to as the "mesophase"), intermediate between their solid crystal form and their isotropic liquid form. The mesomorphic phase of cholesteric liquid crystals is highly colored due to the periodic structure of the phase.
The color exhibiting cholesteric liquid crystals are optically active and are said to be "optically negative". The observed color of the cholesteric liquid crystal in its mesomorphic phase is a function of its chirality or "twist", which will be described in more detail hereinafter. Other liquid crystal compounds such as nematic type liquid crystals are said to be optically positive in the mesomorphic state and do not exhibit a specific color associated with periodic structure. It was heretofore known that one could admix cholesteric type liquid crystals with nematic type liquid crystals to impart chirality and therefore visibility to the mesomorphic phase of the resultant liquid crystal composition; see for example U.S. Pat. Nos. 3,650,603 and 3,984,343.
In general, the prior art temperature sensing and recording devices employing liquid crystals as the temperature indicator have not been entirely satisfactory for a number of reasons. For example, many liquid crystal indicator compositions are unstable and have short shelf lives. This is particularly true of mixtures of liquid crystals with so-called liquid transition point "elevator" or "depressant" additives which are heterogeneous mixtures and which tend to separate on standing. Many of such compositions which include hydrocarbon additives are particularly unstable. The hydrocarbon additives destroy crystallinity of the liquid crystals over prolonged periods of contact. After a relatively short period of time, the liquid transition points of such compositions will have shifted. Therefore, unless the temperature sensing device employing such a composition is used fairly rapidly after its manufacture, inaccurate results will be obtained. Another difficulty heretofore encountered with the prior art compositions is that the temperature range over which a given liquid transition point occurs may be so broad as to render the device useless for certain purposes, for example as a replacement for a medical clinical thermometer where accuracy within plus or minus 0.5.degree. F. is desired. Many prior art temperature sensing devices using liquid crystal temperature indicators are too insensitive to measure such small differences in temperature.
The liquid crystal based temperature sensing compositions of my invention are relatively stable over long periods of time even if the ingredients are relatively impure. They also pass through their liquid transition points very rapidly and over relatively narrow temperature ranges. In other words, the liquid transition points for the liquid crystal compositions of my invention are very sharp and occur at or very near to any specific desired temperature point. In addition, the compositions of my invention exhibit controlled hysteresis and are therefore particularly suited for use as a disposable temperature recorder for clinical medical use. The compositions of the invention also provide a means of color control in visually indicating temperature sensing devices.
Mixtures of cholesteric type liquid crystals with nematic type liquid crystals have heretofore been known in the prior art; see for example U.S. Pat. Nos. 3,650,603; 3,666,881; 3,720,623; 3,923,685; 3,931,041; 3,956,169; 3,973,830; and RE 28,806. Other U.S. patents representative of the prior art are 3,114,830; 3,529,156; 3,716,289; and 3,997,463.